1. Field of the Invention
This invention relates to the reactor vessel internals of a nuclear reactor, and more particularly to apparatus which baffles flow through and about the core of a reactor.
2. Description of the Prior Art
A typical liquid cooled nuclear reactor includes a singular reactor vessel, housing the heat generating reactor core, and a plurality of flow loops through which the reactor coolant fluid is circulated. In each loop, coolant which is heated in the core typically is placed in heat exchange relation with a vaporizable fluid which is used to drive turbine-generator apparatus. The coolant is then recirculated to the reactor vessel. Within the vessel are the reactor vessel internals, the functions of which include support of the core components, such as the fuel assemblies, guidance of reactor coolant flow, and support of core monitoring apparatus. Most of the supported load is transmitted through the wall of a massive core barrel which is spaced from and radially surrounds the reactor core. The main stream of coolant flow generally enters the vessel, passes downward in an annulus formed between the barrel and vessel, and then is turned 180.degree. to flow up through the core and out of the vessel.
In the annular region between the core fuel assemblies and the core barrel is typically located a baffle plates and formers assembly. The baffle plates vertically extend the height of the core, and are positioned adjacent the core outer periphery, so as to enclose and baffle coolant flow through the core. The plates are supported by a plurality of horizontal formers, which are affixed to and supported by the core barrel. In addition to baffling flow through the core, the baffle-former assembly must be positioned close to the peripheral fuel assemblies to maintain a minimum amount of flow in this relatively low power region. If too large a flow area exists, it will detract from the thermal efficiency of the reactor as it bypasses the core and mixes with hot coolant. The baffle-former assembly also provides a coolant containing annular region between the core and the core barrel which shields the barrel from irradiation. Coolant which has entered the vessel, but which has not passed through the core, flows through this annular region. This flow cools the baffles, formers, the core barrel, and the fasteners affixing these components. It is also important that bypass flow through this area not be so large as to unduly reduce reactor thermal efficiency.
There are, however, problems associated with the baffle-former assemblies typically in use. First, the core barrel is typically a relatively thick walled structure due to the massive loads it must support, as compared to the relatively thin walled baffle plates which are not similar load-bearing structures. As a result of temperature changes with changes in core power and power distribution changes throughout an operating cycle, and the thermal gradient between the baffle plates and the core barrel, extremely high loads are placed upon the connecting formers and fasteners due to the differential thermal expansion of the components. The accommodation of the loads in the fasteners, such as bolts or welds, has been unsatisfactory, as discussed in the referenced second Berringer/Machado application.
Second, under assumed and highly unlikely accident conditions, such as a major rupture of the reactor primary system, the reactor will rapidly depressurize. However, the core area will depressurize faster than the baffle-former annular region, potentially resulting in a pressure buildup that could fail the baffle assembly and subsequently the fuel assemblies. This is discussed in more detail in the referenced first Berringer/Machado application.
Third, because the baffle plates are subject to high heat loads and differential expansions, they are not affixed, and merely abut against one another. The plates must meet close tolerances, and the joints between abutting baffle plates must be able to accommodate the thermal expansion. The net result is a design requiring complex, time consuming, and costly manufacturing techniques. Further, some prior art baffle-former assemblies have experienced leakage through gaps formed between adjacent baffle plates, which leakage impinges upon the fuel assemblies and may cause detrimental assembly vibration.
Fourth, the amount of reactor coolant flow which is passed through the baffle-former annular region to cool the components must be carefully controlled. An excessive amount of this flow will significantly detract from the reactor thermal efficiency, as this fluid is mixed with hot fluid exiting the core. Similarly, the amount of coolant flow between the outermost fuel assemblies and the baffle must not be excessive.
Fifth, the baffle plates and formers are typically of a material such as stainless steel. Stainless steel is a parasitic neutron absorber, and therefore also a poor neutron reflector, which further results in decreased reactor efficiency. Also, because of the orientation of the baffle plates, they may exhibit a tendency to bow, thereby changing the flow baffling characteristics and potentially contacting the peripheral fuel rods, which would cause local flow starvation and excessively high fuel rod temperatures.
Some of thse concerns have been addressed in U.S. Pat. No. 3,607,637, issued Sept. 21, 1971, to Robert C. Marshall, hereinafter referred to as the Marshall patent. The Marshall patent provides a nuclear reactor including a plurality of seal plates fastened to a fluid confining means surrounding the core, such as a core barrel, and having the inner edges of the plates adjacent, but spaced from, the outer fuel elements of the core. The plates are longitudinally positioned at substantially evenly spaced locations intermediate the ends of the core. The patent also contemplates the use of neutron reflectors vertically mounted on the seal plates. The Marshall patent, therefore, alleviates many of the above problems by elimination of the baffle (referred to as a "shroud" in that patent).
However, there remain concerns with the arrangement as provided in the Marshall patent. The most significant is that there remains the potential for contact of the seal plates (referred to as baffle plates throughout this application), with the peripheral fuel rods. Such contact of a seal plate and a heat generating fuel rod for a long duration could result in flow starvation, excessive localized heating, and fretting of the rod, potentially resulting in rod failure. Further, it is well recognized that under irradiation fuel rods may bow and are also subject to vibration in the high velocity coolant stream. Therefore, there may be intermittent contact among the outermost fuel rods and the seal plates, potentially resulting in increased fretting or even rupture of the rods. Merely spacing the edges of the seal plates farther from the core could alleviate this, but such an arrangement would compromise the flow control, allowing excessive coolant to bypass the core region.
Also, spacing of the seal plates at substantially even increments along the core length is likely not to be the best arrangement from a thermal-hydraulic standpoint. Coolant flow through a core is subject to varying conditions along the assembly length. Most notably, the pressure drop is significant as the coolant flows about and through the assemblies, resulting in a lower pressure at the core outlet. Further, and of great significance in those reactors with fuel assemblies including a plurality of fuel rods laterally supported by grid structures, is the localized effect of the coolant flow as it approaches and passes about a fuel assembly grid structure location. The grid location presents a significant resistance to flow and therefore a large pressure drop. The flow will therefore tend to discharge radially as it approaches a grid. With a mismatch in the elevations of the grids and the elevations of the seal plates of the Marshall patent, such as a grid located between two seal plates, excessive lateral and vortex flow may be experienced in the volume between the consecutive seal plates. This could detrimentally cause flow starvation at the fuel assembly due to the lateral flow component and increased asembly vibration and fretting.
It is therefore desirable to provide an arrangement which overcomes the stated deficiencies of the prior art and the Marshall patent. Such an arrangement should still meet the main functional criteria of baffling flow into and properly about the core, as well as maintaining an acceptably low volume of bypass flow.